A central problem in the design and implementation of data transmission methods is simultaneous transmission and reception of signals from several simultaneous users in such a way that the signals interfere with one another as little as possible. Because of this and the transmission capacity used, various transmission protocols and multiple access methods have been developed, the most common of them especially in mobile phone communication being FDMA and TDMA methods and, recently, a CDMA method as well.
CDMA is a multiple access method based on a spread spectrum technique, and recently it has been put into use in cellular radio systems in addition to previously used FDMA and TDMA. In a CDMA method, a narrow-band data signal of a user is multiplied to a relatively broad band by a spreading code having a much broader band than the data signal. Bandwidths used in known test systems include e.g. 1.25 MHz, 10 MHz and 25 MHz. The multiplication spreads the data signal over the entire band to be used. All the users transmit simultaneously in the same frequency band. On each connection between a base station and a mobile station, a different spreading code is used, and the signals of the users can be separated from one another in the receivers on the basis of the spreading code of the user.
When a TDMA multiple access method is applied in a data transmission system, several frequencies are usually in use, each frequency being divided into time slots in which signals from different users are placed. Each user is thus assigned a time slot of his own. Since the frequency band reserved for the system is usually limited, the frequencies used must be repeated in cells located within a certain distance.
In a data transmission method in which a FDMA multiple access method is applied there are usually several frequencies in use, and each user communicates in his own narrow frequency band.
The different directions of transmission are typically at their own frequencies in all multiple access methods, although this is not compulsory.
For mobile phone networks, various standards have been designed in which different multiple access methods are used on the air interface. With an increase in the amount of mobile telecommunication, the number of systems used has risen. The systems are maintained by various operators, but one and the same operator may also maintain various systems. Local coverage areas of systems utilizing different multiple access methods may overlap, but until now they must have had different frequencies in order for their transmissions not to interfere with one another.
It is previously known that one and the same receiver is able to receive signals transmitted by different multiple access methods. The operation of these `dual mode` receivers is based on the fact that signals transmitted by different multiple access methods are at different frequencies. Such receivers have been designed, for example, for CDMA/FDMA and TDMA/FDMA environments.